Solid state fluid level sensor

CROSS-REFERENCE TO RELATED APPLICATION

0001This patent application is a divisional of U.S. patent application Ser. No. 10/886,558, filed Jul. 9, 2004, now U.S. Pat. No. 7,373,817 the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

00021. Field of the Invention

0003The present invention relates to sensors for detecting the presence of a fluid, automatic systems for actuating pumps in response to detecting a fluid level and sensors mounted in the bottom of a boat bilge tank activating a bilge pump when the bilge fluid level reaches a preset distance above the bottom of the bilge tank.

00042. Discussion of the Prior Art

0005In the past, bilge pumps have been activated manually or by mechanical float type switches with mercury or point contacts to complete an electrical circuit activating a pump. Pressure switches have also been used. These prior art switches worked adequately when initially installed. Over time, however, bilge debris and other sources of contamination often prevented the mechanical components from moving as intended, causing switch failure. In addition, prior art bilge pump activation switches typically wore out several times during the life of a boat and, being located in a boat's nether regions, were difficult to access for repair and replacement.

0006Many fluid level or fluid proximity detectors of the prior art employed electrical switches actuated when a conductor, such as a body of water, moved into close proximity to the detector or sensor. U.S. Pat. Nos. 3,588,859; 3,665,300; 4,800,755 and 4,875,497 disclose such detectors. U.S. Pat. No. 5,017,909, discloses a proximity detector used as a liquid level detector for receptacles in vehicles.

0007Other applications for liquid level detectors included bilge-pumping systems for ships. A bilge pumping system must be activated before the accumulated water reaches an excessive level. Prior art mechanisms for detection of an excessive bilge water level employed mechanical floatation systems, causing a switch to be actuated whenever the water reached such an undesired level. Bilge fluid or water eventually renders mechanical level sensing systems inoperative in part because bilge fluid can contain many forms of corrosive waste. Replacing failed parts of a bilge level sensing system can be very expensive and troublesome, since a skilled technician must enter the bilge to perform the work.

0008Many electronic proximity detection systems have been proposed in searching for a solution to this messy, expensive problem. By way of example, Smith et al (U.S. Pat. No. 4,881,873) discloses a capacitive level sensor for a bilge pump including a sensor plate 40 positioned in a bilge at a position selected for pump actuation. The bilge water is sensed as a dielectric, in a manner of speaking, and so the sensor is susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the area around the sensor, and sloshing bilge water is likely to cause the bilge control to actuate when the bilge level does not require pumping.

0009Gibb (U.S. Pat. No. 5,287,086) also discloses a capacitive level sensor for a bilge pump including a capacitive sensor plate 79 positioned in a bilge at a position selected for pump actuation. The sensor is contained within a sealed housing 32 to keep bilge water away from the sensor and other circuitry. Here again, bilge water is sensed as a dielectric, in a manner of speaking, and so the sensor is susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the area around the sensor, and sloshing bilge water is likely to cause the bilge control to actuate when the bilge level does not require pumping.

0010Santiago (U.S. Pat. No. 4,766,329) discloses a solid-state two level sensor for a bilge pump including a high water level probe and a low water level probe, both positioned in a bilge at positions selected for pump actuation. The probes are in contact with the bilge water, and so the probe sensors are susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the probes.

0011Farr (U.S. Pat. No. 5,238,369) discloses a system for liquid level control including upper and lower capacitive level sensors 10, 18 having positions selected for pump actuation. This reference is silent on the need to keep bilge water away from the sensors, but the bilge water is sensed as a dielectric, in a manner of speaking, and so the sensor is susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the area around the sensors.

0012The applicant has licensed a Field Effect sensor patent to Caldwell (U.S. Pat. No. 5,594,222) on a “touch sensor” used to detect whether a user presses a virtual button; this sensor is referred to as a “touch sensor.” While the patent discloses the electromagnetic properties of Field Effect “touch” sensing, it is silent on how such technology might be employed in a sensor system for detecting a fluid/air interface or for automated bilge pump actuation.

0013There is a need, therefore, for a system for sensing liquid level and liquid level control that overcomes the problems with prior art sensors and systems, permitting installation of a reliable, inexpensive fluid level sensing system which is unlikely to require maintenance or cleaning in the bilge. It would be highly desirable to have a new and improved proximity detection system which is highly reliable and relatively inexpensive to manufacture. Such a proximity detection system should be highly sensitive and possess a wide range of applications.

SUMMARY OF THE INVENTION

0014The fluid level control and sensor system of the present invention comprises a fluid tight housing or container and a circuit board with electrodes and interconnect patterns assembled with components to create an electric field having arc shaped patterns and sense changes using the field effect principle.

0015The housing or container holds first and second electrode patterns in a vertical orientation and has ribs on the housing's sides to allow debris found in a bilge to slough off. By careful selection of materials, the container can resist biological attack (e.g., fungus or algae) and prevent fouling from other materials that might stick to the container otherwise. In the vertical position, gravity also helps to allow the anticipated contamination to slough off.

0016The field effect is described elsewhere in U.S. Pat. No. 5,594,222 and others assigned to TouchSensor, LLC, which are typically used in large appliance applications for operator input. A similar principle is adapted to detect liquids in close proximity to the sensor, even when isolated from the liquid by a physical barrier, such as a tank wall or molded container. This technique rejects common mode contamination to the sensor and, through proper tuning of the device, allows the presence or absence of a liquid to be detected. Since this is a solid state device in close proximity to the detector and it is also low impedance, it is also very tolerant of electrical noise in the marine environment.

0017The electrode design can have geometries ranging from parallel plates to concentric rings of various sizes and geometric shapes. The design of the electrodes is determined by the materials of construction, thickness, composition of the liquid and other considerations.

0018Since the fluid level control and sensor system is submerged when active and passing current, an internal current switching device (e.g. a Field Effect Transistor (FET)) adapted to pass twenty amps is easily cooled.

0019Electronics to support the sensor optionally include components allowing control of devices demanding twenty amps of current without the addition of a separate relay. Through current scalping and other techniques, the bilge pump control system of the present invention operates through two wires or can have a separate third wire to provide power. The fluid level control and sensor system can be implemented with or without a microprocessor.

0020The bilge pump assembly consists of a housing, circuit board, components and wiring harness. In operation, the bilge pump sensor is installed in-line between the pump and a power supply. The circuit draws its own power from the power mains supply without activating the pump. Without bilge liquid present near the sensor electrodes, a sensing IC is at a first state, “off.” With a liquid present near the sensor electrodes, the IC changes state to “on” and the circuitry allows connection of power to the bilge pump. The pump is activated until the liquid level goes below the sensor electrodes. The sensing IC changes state to “off” and the power to the pump is interrupted, causing the pump to stop. The sequence is repeated whenever liquid comes in proximity to the sensor. An optional microprocessor allows control of on-off timing and other time management operations provide a stable pump operation without rapid changes from the “on” to the “off” state (e.g., due to an instability referred to as “sloshing”). Sloshing may vary in amplitude depending on the length of craft and bilge tank and the rocking motion of the craft.

0021The bilge pump controller of the present invention includes a field effect sensor comprising an active, low impedance sensor on a dielectric substrate. The sensor has a first conductive electrode pad and a second conductive electrode which substantially surrounds the first electrode in a spaced apart relationship. The first electrode pad has a closed, continuous geometric shape and both electrodes are attached to the same surface of the substrate. An active electrical component is placed in close proximity to the electrodes.

0022The sensor is used to replace conventional switches and is activated when bilge fluid or water contacts or comes into close proximity with the substrate. The sensor is used to turn an electric pump motor on or off. The field effect sensor design operates properly with liquids present on the substrate and in the presence of static electricity, and is well-suited for use in an environment where water, grease and other liquids are common, such as boat bilges or other sea-going applications.

0023Electrodes are attached to the back surface of a substrate, opposite the front or “wet” surface, thereby preventing contact between the electrodes and the controlled fluid (e.g., bilge water). Since the sensor electrodes are not located on the wet surface of the substrate, the sensor is not damaged by scratching, cleaning solvents or any other contaminants which contact the substrate.

0024The cost and complexity of the sensor is reduced since a relay or switch is not required.

0025In the preferred form, an oscillator is electrically connected to the inner and outer electrodes through gain tuning resistors and delivers a square-wave like signal having a very steep slope on the trailing edge. An electric potential is developed at each electrode and, consequently, an electric field is generated at each of the electrodes. The oscillator signal creates an arc shaped transverse electric field between the outer electrode and the center electrode. The electric field path is arc-shaped and extends through the substrate and past the front surface, projecting transversely to the plane of the substrate. The inner and outer electrode signals are applied as common mode signals to the inputs of a differential sensing circuit and, when the difference in response between the inner and outer electrodes is great enough, the sensing circuit changes state (e.g., from high to low). Accordingly, the differential sensing circuit detects and responds to differences in electrical potential between the inner and outer electrodes in response to the introduction of a stimulus in proximity to the inner electrode, the outer electrode, or both, such as when the substrate is touched by the controlled fluid. Such a differential sensing circuit provides for the rejection of common mode signals (i.e., signals that would tend to affect both electrodes approximately equally), such as contaminants to the sensor.

0026In the preferred form, an active electrical component preferably configured as a surface mount application specific integrated circuit (ASIC), is located at each sensor. Preferably, the ASIC is connected to the center pad electrode and to the outer electrode of each sensor. The ASIC acts to amplify and buffer the detection signal at the sensor, thereby reducing the difference in signal level between individual sensors due to different lead lengths and lead routing paths. A plurality of sensors may be arranged on the substrate.

0027The above and still further features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings, wherein like reference numerals in the various figures are utilized to designate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

0028<figref idref="DRAWINGS">FIG. 1</figref> is a side view, in elevation, of a bilge fluid level control and sensor system, in accordance with the present invention.

0029<figref idref="DRAWINGS">FIG. 2</figref> is a partial cross section, in elevation, of the fluid level control and sensor system of <figref idref="DRAWINGS">FIG. 1</figref>, in accordance with the present invention.

0030<figref idref="DRAWINGS">FIG. 3</figref> is a back side view, in elevation, of the primary, back or dry surface of a printed wiring board assembly for the fluid level control and sensor system of <figref idref="DRAWINGS">FIGS. 1 and 2</figref>, in accordance with the present invention.

0031<figref idref="DRAWINGS">FIG. 4</figref> is a schematic diagram of the printed wiring board assembly for the fluid level control and sensor system of <figref idref="DRAWINGS">FIGS. 1</figref>, <b>2</b> and <b>3</b>, in accordance with the present invention.

0032<figref idref="DRAWINGS">FIG. 5</figref> is a diagram illustrating a field effect fluid level sensor system showing the arc-shaped field passing through the fluid, in accordance with the present invention.

0033<figref idref="DRAWINGS">FIG. 6</figref> is a diagram drawn to scale and illustrating the component side layout of conductive traces on a printed circuit board including a balanced pad sensor electrode pattern.

0034<figref idref="DRAWINGS">FIG. 7</figref> is an edge view, in elevation, illustrating the conductive traces on the printed circuit board of <figref idref="DRAWINGS">FIG. 6</figref> including the balanced pad sensor electrode pattern, as seen from the side.

0035<figref idref="DRAWINGS">FIG. 8</figref> is an edge view, in elevation, of an alternative two-sided embodiment illustrating the conductive traces on a printed circuit board including the balanced pad sensor electrode pattern, as seen from the side.

DESCRIPTION OF THE PREFERRED EMBODIMENT

0036Referring to the exemplary embodiment illustrated in <figref idref="DRAWINGS">FIGS. 1-5</figref>, a bilge or fluid containment vessel <b>10</b> is bounded by a fluid tight wall or surface <b>18</b> and at times contains bilge water or some other fluid <b>12</b>. The fluid level in the bilge rises and falls and the fluid level is measurable over a selected dimension such as that shown by the vertical scale in <figref idref="DRAWINGS">FIG. 1</figref>. As the fluid level rises or falls, a fluid/air interface <b>16</b> can be sighted or measured along the fluid level scale. In the typical marine application, bilge <b>10</b> contains fluid <b>12</b> such as waste water or seawater that leaks through the hull or deck, and when the bilge fluid level is excessively high (e.g., at a selected upper or trigger level <b>16</b>H), fluid <b>12</b> must be pumped out, usually with an electrically powered pump (not shown) that is selectively energized when the excessively high fluid level <b>16</b>H is detected. The fluid level is sensed while pumping progresses and the pump is turned off when the level of fluid <b>12</b> is low enough (e.g., at a selected lower or turn-off level <b>16</b>L).

0037As shown in <figref idref="DRAWINGS">FIGS. 1</figref>, <b>2</b> and <b>3</b>, fluid level control and sensor system <b>20</b> of the present invention comprises a fluid tight housing or container <b>24</b> and a printed wiring board assembly <b>22</b> with electrodes <b>50</b>, <b>52</b>, <b>60</b> and <b>62</b> and interconnect patterns assembled with components to create an electric field having arc shaped patterns and sense changes using the “field effect” principle.

0038Referring to <figref idref="DRAWINGS">FIGS. 4 and 5</figref>, the field effect is generated and changes in the field are sensed using patterns of conductive traces or electrodes <b>50</b>, <b>52</b> which create arc shaped electric fields <b>54</b> projecting transversely and passing through substrate <b>30</b>. When a rising fluid/air interface <b>16</b> is sensed, the arc shaped electric fields <b>54</b> pass through the fluid <b>12</b> rather than the air above the fluid/air interface. The operation of the sensor system <b>20</b> will be described in greater detail below.

0039In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, printed wiring board assembly <b>22</b> is disposed within a cavity <b>26</b> of housing or container <b>24</b> and, during assembly, is sealed inside to provide a fluid proof barrier of substantially uniform thickness. Printed wiring board or circuit board <b>30</b> is a dielectric planar substrate of substantially uniform thickness having a back side (shown in <figref idref="DRAWINGS">FIGS. 2 and 3</figref>) carrying the components and opposite a front side. The sensor system <b>20</b> is connected to a mains supply and an electric pump (not shown) via a two cable wire assembly <b>32</b> preferably comprising at least first and second sixteen gauge (16 AWG) wire segments.

0040The electrical components in the sensor embodiment illustrated in <figref idref="DRAWINGS">FIGS. 2</figref>, <b>3</b> and <b>4</b> include a pre-programmed integrated circuit (IC) eight bit micro-controller also identified as U<b>4</b>, which is connected to and responsive to first and second touch switch ICs, <b>36</b>, <b>37</b> also identified as U<b>2</b> and U<b>3</b>, respectively. A sensor system power supply includes voltage regulator <b>38</b>, also identified as U<b>1</b>, and the mains supply voltage is also controlled by a pair of diodes including series connected diode <b>46</b> and shunt connected zener diode <b>44</b> providing power to a voltage regulator <b>38</b>.

0041In the illustrated embodiment, the bilge water pump actuation and supply comes from a large switching N channel Field Effect Transistor (NFET) <b>42</b>, the mains supply voltage (e.g., 12 VDC) is connected through connector J<b>1</b> to the drain and the current, when switched on to actuate the pump, passes through the source and through connector J<b>2</b> to the pump.

0042As best seen in <figref idref="DRAWINGS">FIG. 3</figref>, a first sensor electrode pattern <b>49</b> includes a substantially rectangular upper center electrode pad <b>50</b> including a plurality of vertical conductive traces separated by approximately equal width sections of non-conductive PCB surface, where each conductive trace is connected at its upper and lower ends by a surrounding conductive trace, all connected to resistor R<b>5</b>. Upper center electrode pad <b>50</b> is not quite completely encircled by upper outer electrode <b>52</b> which is connected to upper touch sensor ASIC <b>36</b> through resistor R<b>6</b>. Both inner pad <b>50</b> and outer electrode <b>52</b> are at least partly encircled by a perimeter of solid conductive trace material to provide a ground ring (not shown). The ground ring can be configured to influence sensitivity and directionality of outer electrode <b>52</b>; if the ground ring is situated too closely to outer electrode <b>52</b>, however, the sensor's differential mode is lost and the sensor behaves in a single ended fashion.

0043A second sensor electrode pattern <b>59</b> is disposed in a vertically aligned orientation below the first sensor electrode pattern <b>49</b> and includes a substantially rectangular lower center electrode pad <b>60</b> including a plurality of vertical conductive traces separated by approximately equal width sections of non-conductive PCB surface, where each conductive trace is connected at its upper and lower ends by a surrounding conductive trace, all connected to resistor R<b>7</b>. Lower center electrode pad <b>60</b> is not quite completely encircled by lower outer electrode <b>62</b> which is connected to lower touch sensor ASIC <b>37</b> through resistor R<b>8</b>. Both pad <b>60</b> and outer electrode <b>62</b> are also at least partially encircled by a second perimeter of solid conductive trace material to provide a second ground ring (not shown).

0044In accordance with the present invention, the field effect principle is adapted to detect liquid <b>12</b> when in close proximity to sensor <b>20</b>, even when isolated from the liquid by a physical barrier such as a tank wall or molded container (e.g., <b>24</b>). This technique rejects common mode contamination to the sensor and, through proper tuning of the device, allows the presence or absence of liquid or fluid (e.g., bilge water <b>12</b>) to be detected. Since the touch sensors ASICs <b>36</b>, <b>37</b> are solid state devices in close proximity to the electrode pads and also of low impedance, they are also very tolerant of electrical noise in the marine environment.

0045The electrode design can have geometries ranging from parallel plates to concentric rings of various sizes and geometric shapes. The design of the electrodes (e.g., <b>50</b>, <b>52</b>, <b>60</b> and <b>62</b>) is determined by the materials of construction, thickness, composition of liquid <b>12</b> and other considerations, as will be appreciated by those with skill in the art.

0046In the illustrated embodiment, the electronics supporting the sensor include components allowing control of devices (e.g., a pump) demanding 20 amps of current without the addition of a separate relay. Since fluid level control and sensor system <b>20</b> is submerged when active and passing current, heat sinking to the bilge fluid <b>12</b> allows an internal current switching device (e.g. NFET <b>42</b>) and a twenty amp supply circuit to be easily cooled.

0047Through current scalping and other techniques, the fluid level control and sensor system <b>20</b> operates through two wires or can have a separate third wire (not shown) to provide power. Current scalping or scavenging utilizes the error output of voltage regulator <b>38</b> as an input to microprocessor <b>34</b> which is programmed to turn off FET switch <b>42</b> for a short interval during which shunt storage capacitor C<b>1</b> charges back to 12 volts. This current scalping method permits an “on-time” for FET switch <b>42</b> of at least 97%.

0048Fluid level control and sensor system <b>20</b> can be implemented with or without a microprocessor (not shown).

0049Housing or container <b>24</b> supports and protects first and second electrode patterns <b>49</b> and <b>59</b>, in a vertical orientation and optionally has one or more external ribs transversely projecting vertically aligned elongated features <b>27</b> on the housing's sides to allow bilge debris to slough off or away from housing <b>24</b> as the fluid level falls during pumping. Housing <b>24</b> is preferably made of inert materials such as ABS, polypropylene or epoxy to resist biological attack (e.g., fungus or algae) and prevent fouling from other substances that might otherwise tend to stick to the housing or container sidewalls. In the vertical orientation or position shown in <figref idref="DRAWINGS">FIG. 1</figref>, gravity also helps to allow the anticipated contamination to slough off.

0050In operation, the bilge pump sensor <b>20</b> is installed in-line between the pump and a power mains supply using wire assembly <b>32</b>. The circuit draws its own power from the power mains supply through connector J<b>1</b> without activating the pump. Without bilge liquid present near the sensor electrodes, each of the sensing ICs <b>36</b>, <b>37</b> is at a first state, “off.” With liquid <b>12</b> present near the sensor electrodes, the sensing ICs <b>36</b>, <b>37</b> change state to a second state, “on”, and the circuitry connects power through FET <b>42</b> to the bilge pump. The pump remains activated until the sensed liquid level <b>16</b> goes below the sensor electrodes <b>49</b> and <b>59</b>. The sensing ICs <b>36</b>, <b>37</b> then change state to “off” and the power to the pump is interrupted, causing the pump to stop. This sequence is repeated whenever the fluid surface <b>16</b> comes into proximity with the sensor system <b>20</b>. An optional microprocessor allows control of on-off timing and other time management operations to provide a stable pump operation without rapid changes from the “on” to the “off” state (e.g., due to an instability referred to as “sloshing”). Sloshing may vary in amplitude depending on the length of craft and bilge tank and the rocking motion of the craft.

0051The bilge pump controller or sensor system <b>20</b> of the present invention includes at least one field effect sensor comprising an active, low impedance sensor attached to only one side of dielectric substrate <b>30</b> and is used to replace conventional switches. The field effect sensor design operates properly with liquids present on the substrate and in the presence of static electricity, and is well-suited for use in a marine environment where water, grease and other liquids are common, such as boat bilges or other sea-going applications.

0052As shown in <figref idref="DRAWINGS">FIG. 5</figref>, the sensor's printed wiring board assembly <b>22</b> may be molded or fabricated into the bilge sidewall <b>18</b> rather than being separately encapsulated in housing <b>24</b>, with the front or “wet” side of the PC board <b>30</b> facing the interior of the bilge <b>10</b>.

0053Preferably, sensor electrode patterns <b>49</b>, and <b>59</b> are attached to the back surface of PCB substrate <b>30</b>. The back surface of the substrate is opposite the front or “wet” surface, thereby preventing contact of the electrodes by the controlled fluid (e.g., bilge water). Since the sensor electrodes are not located on the front surface of the substrate, the sensor is not damaged by scratching, cleaning solvents or any other contaminants which contact the front surface of the substrate. Furthermore, the cost and complexity of the sensor is reduced since a switch is not required.

0054Preferably, an active electrical component, such as a surface ASIC (e.g., <b>36</b>) is located at each sensor and connected between the center electrode (e.g., <b>50</b>) and the outer electrode (e.g., <b>52</b>) of each sensor. The ASIC acts to amplify and buffer the detection signal at the sensor, thereby reducing the difference in signal level between individual sensors due to different lead lengths and lead routing paths.

0055The Integrated circuit connected to the field effect sensor electrodes is an active device and, in the illustrated embodiment, is preferably configured as ASIC operating in the manner described in U.S. Pat. No. 6,320,282, to Caldwell, the entire disclosure of which is incorporated herein by reference. As described above, a simple field effect cell has two electrodes (e.g., <b>50</b>, <b>52</b>), an ASIC (e.g., <b>36</b>) and two gain tuning resistors (e.g., R<b>5</b> and R<b>6</b>). The pin-out for the TS-100 ASIC of the invention is similar to that illustrated in <figref idref="DRAWINGS">FIG. 4</figref> of the '<b>282</b> patent, but the pin-outs vary slightly. The TS-100 ASIC is available from Touch Sensor, LLC. Specifically, for the TS-100 ASIC shown in this application, the input power (Vdd) connection is on pin <b>1</b>, the ground connection is on Pin <b>2</b>, the sensor signal output connection is on pin <b>3</b>, the outer electrode resistor (e.g., R<b>6</b>) is connected to pin <b>4</b>, the “oscillator” connection is at pin <b>5</b> and the inner pad electrode resistor (e.g., R<b>5</b>) is connected to pin <b>6</b>. Optionally, an ASIC can be configured to eliminate the need for gain tuning resistors (e.g., R<b>5</b> and R<b>6</b>) by making the gain tuning adjustments internal to the ASIC.

0056The sensitivity of the field effect sensor or cell is adjusted by adjusting the values of gain tuning resistors (e.g., R<b>5</b> and R<b>6</b>). The sensor of the present invention can be adapted for use in a variety of applications and the gain resistors (e.g., R<b>5</b> and R<b>6</b>) can be changed to cause a desired voltage response. The sensor of the present invention is like other sensors in that the sensor's response to measured stimulus must be tuned or calibrated to avoid saturation (i.e., from gain/sensitivity set too high) and to avoid missed detections (i.e., from gain/sensitivity set too low). For most applications, a gain tuning resistor value which yields a sensor response in a linear region is preferred. The tuning or calibration method typically places the sensor assembly in the intended sensing environment and the circuit test points at the inputs to the decision circuit (e.g., points <b>90</b> and <b>91</b> as seen in <figref idref="DRAWINGS">FIG. 4</figref> of Caldwell's '282 patent) are monitored as a function of resistance. The resistance value of the gain tuning resistors (e.g., R<b>5</b> and R<b>6</b>) are adjusted to provide an output in the mid-range of the sensor's linear response.

0057While other electrode patterns are suitable for this bilge pump control application, the illustrated electrode patterns <b>49</b> and <b>59</b>, as shown in <b>3</b>, are each balanced. “Balanced” as used here, means that the conductive trace area of the inner electrode (e.g., center pad <b>50</b>) is equal (or as equal as possible within PCB manufacturing tolerances) to the area of its corresponding outer electrode ring (e.g., outer electrode <b>52</b>).

0058The applicants have discovered that the illustrated balanced pad electrode design provides improved noise or electromagnetic interference (EMI) immunity and works exceptionally well for sensing the presence of a fluid such as water.

0059The EMI immunity appears to stem from a common mode rejection of spurious noise or interference signals. This “common mode” rejection is attributable to the equal area of the center pad and the outer ring electrode, which appear to be affected by spurious noise or interference signals substantially equally, and so when one electrode's signal is subtracted from the other electrode's signal, the common noise/interference signals cancel one another.

0060In the embodiment illustrated in <figref idref="DRAWINGS">FIG. 3</figref>, each balanced center pad is substantially rectangular having a horizontal extent of approximately twelve millimeters (mm) and a vertical extent of nine mm. As can be seen from <figref idref="DRAWINGS">FIG. 3</figref>, each center electrode pad (e.g., <b>50</b> and <b>60</b>) includes a plurality of vertical conductive traces (each approx. one mm in width) separated by approximately equal width sections of non-conductive PCB surface, where each conductive trace is connected at its upper and lower ends by a surrounding conductive trace material, all connected to a resistor (e.g., R<b>5</b> for upper center pad <b>50</b> and R<b>7</b> for lower center pad <b>60</b>). Each center electrode pad (e.g., <b>50</b> and <b>60</b>) is not quite completely encircled by an outer electrode (e.g., <b>52</b> for upper center pad <b>50</b> and <b>62</b> for lower center pad <b>60</b>) which is approximately 1.5 mm in width and is connected to an ASIC (e.g., <b>36</b> for upper outer electrode <b>52</b> and <b>37</b> for lower outer electrode <b>62</b>) through a gain tuning resistor (e.g., R<b>6</b> for upper outer electrode <b>52</b> and R<b>8</b> for lower outer electrode <b>62</b>).

0061An alternative electrode pattern embodiment is illustrated in <figref idref="DRAWINGS">FIGS. 6 and 7</figref>; <figref idref="DRAWINGS">FIG. 6</figref> is a diagram drawn to scale and illustrating the component side layout of conductive traces on a printed circuit board including a balanced pad sensor electrode pattern <b>69</b>, and <figref idref="DRAWINGS">FIG. 7</figref> is an edge view, in elevation, illustrating the conductive traces of balanced pad sensor electrode pattern <b>69</b>, as seen from the side of the PCB (e.g., such as PCB <b>30</b>). As noted above, a balanced pad or electrode pattern has a conductive trace area for the inner electrode (e.g., center pad <b>70</b>) that is equal (or as equal as possible within PCB manufacturing tolerances) to the area of its corresponding outer electrode ring (e.g., outer electrode <b>72</b>). Both pad <b>70</b> and outer electrode <b>72</b> are also optionally encircled by a perimeter of solid conductive trace material to provide a ground ring <b>76</b>.

0062The optional ground ring <b>76</b> is useful for reducing the effects of the boat or bilge material on the operation of the sensor <b>20</b>. When a ground ring <b>76</b> is included, sensor <b>20</b> may be installed in either a conductive (e.g. aluminum or steel) bilge or a dielectric (e.g., fiberglass) bilge with negligible effects on sensor performance.

0063An alternative electrode pattern embodiment is illustrated in <figref idref="DRAWINGS">FIG. 8</figref>, a diagram illustrating an edge view of a two-sided layout of conductive traces on a printed circuit board including a balanced pad sensor electrode pattern <b>79</b>, as seen from the side of the PCB (e.g., such as PCB <b>30</b>). As above, a balanced pad or electrode pattern has a conductive trace area for the inner electrode (e.g., center pad <b>80</b>) carried on one side of the PCB that is equal (or as equal as possible within PCB manufacturing tolerances) to the area of its corresponding outer electrode ring (e.g., outer electrode <b>82</b>) on the opposite side of PCB <b>30</b>. Both pad <b>80</b> and outer electrode <b>82</b> are at least partially encircled by a perimeter of solid conductive trace material to provide a ground ring <b>86</b> which is spaced apart from outer electrode <b>82</b>.

0064An alternative embodiment with microprocessor control can be programmed to provide delays for pump turn-on and turn-off. The micro-controller has been programmed to read the lower level pad <b>60</b> and the upper level pad <b>50</b>. Both pads must be activated for at least 256 consecutive reads before the bilge pump is turned on. The interval between reads is 0.0011 seconds, resulting in 0.286 seconds of total time of continuous reads with both pads active before turning on the pump. If at any time, either sensor is not active, the read is canceled and no pump enabling action is taken. —During periods when the pump is not active, the controller puts itself to sleep for 2.5 seconds and then wakes up and reads both upper and lower pads <b>50</b>, <b>60</b>. Thus, the time between water level readings is about 2.78 seconds. This pre-programmed algorithm would have a high probability of not turning on the pump if the water is merely sloshing around in bilge <b>10</b>, but gives fairly quick response. This pre-programmed algorithm is preferably stored in a memory and operates in a manner similar to the principals of fuzzy logic.

0065Other methods may be used to overcome “sloshing” Various electronic methods include using simple timing circuits like “lone shots”, “pulse switching and timing circuits”, and “low pass filtering”. None of these would be as economical as using the single chip micro-computer incorporating the entire control function in simple circuit using a micro, however.

0066A mechanical anti-sloshing control method includes configuring a plastic chamber or shield around the sensing area and allowing water to slowly flow in and out of the chamber and sensing area thereby preventing water level from changing very rapidly in the area of the sensor. This damped flow method would not prevent sensors from switching on/off rapidly when the water level was at the very edge of activation, where slight movements of water or electrical noise would cause sensor oscillation. The mechanical anti-slosh chamber could be coupled with a simple electronic filter circuit such as a low pass and comparator, but this would require more plastic than the illustrated design. The mechanical method of slosh proofing could also be used in conjunction with our current micro-controller design. The anti-sloshing chamber may create problems if the water entry points were to get clogged due to dirt and/or debris.

0067The illustrated micro controller solution results in the simplest, smallest, cheapest, lowest energy consumption result with maximum flexibility and reliability, and so is preferable to non-micro solutions or mechanical methods.

0068Having described preferred embodiments of a new and improved method, it is believed that other modifications, variations and changes will be suggested to those skilled in the art in view of the teachings set forth herein. It is therefore to be understood that all such variations, modifications and changes are believed to fall within the scope of the present invention as defined by the appended claim.